In recent years, the need for low-sulfur fuel oil has ever been increasing for prevention of environmental pollution. On the other hand, as crude oil has become heavy globally, there is a tendency that crude oil having a high content of sulfur, asphaltene, metal and the like should be treated, and the conditions of hydrotreating atmospheric residual oil or vacuum residual oil to obtain low-sulfur fuel oil have been getting stricter. The prolonged demand structure involving a shortage of middle cuts is also a background of this tendency. As a result, improvements in activity and deactivation of a hydrotreating catalyst have been studied intensively, aiming at increase of production of low-sulfur fuel oil by hydrotreating heavy hydrocarbon oil.
Heavy hydrocarbon oil often contains metal compounds, mainly nickel, vanadium, and the like. If such heavy hydrocarbon oil is used as a raw material in a catalytic treatment step, these metal compounds deposit on the catalyst to diminish the catalyst activity and to shorten the catalyst life. Therefore, the metal content should be removed from the heavy hydrocarbon oil before catalytical treatments.